A vacuum chamber from a pressure cooker

[Allan] needed a small vacuum chamber to get all the air out of clear casting resin. Degassing is a simple step in casting that improves the finished product immensely. The problem, though, is building a vacuum chamber. [Allan]’s chamber seems easy enough to build, and pulls enough air out to get to 0.1 atmospheres.

After a hole was drilled in the side of the pressure cooker, [Allan] installed a 15mm “speedfit” plastic tank connector. The seal around the connector is neoprene self-adhesive foam. This foam was also taped around the lip of the pressure cooker for the top.

A thick-walled pressure cooker is more than capable of handling the outside pressure when under vacuum, but [Allan] cautions against using acrylic plastic for the top. Acrylic has the tendency to fail catastrophically, so he used a thick sheet of Lexan. Check out the demo video of [Allan] sucking the air out of shaving cream after the break.

I’ve made the same sort myself. I doubt he’s going to get a great ultimate vacuum with the foam seal, but as an off the shelf solution it’s pretty clever. I ended up casting my own from PU rubber, which works but has started to crack and degrade. I’ll have to do another in silicone.
I’d used 12mm polycarbonate on smaller aperture chambers before, but on something as wide as the top of a pressure-cooker I used 25mm as the 12mm already bowed slightly on an aperture about half the width. Less brittle, but I still don’t want to tempt fate using material that’s too thin.

Also you don’t really need a flange-fitting. I drilled and tapped my holes for screw-in fittings. Just needs a little PTFE tape and it holds fine.

On the first few pumpdowns, the seal wasn’t actually the limiting factor. I (feel free to shudder at this point) use PVC pipe on my primary reservoir that I really don’t want to stress too much. Also, the compressor I’m using (a standard shop compressor) is really not designed for this – The one-way flap valves have a “good enough” seal against pressure but are not great at vacuum.

That’s basically the same thing, but done on purpose. Looks similiar, at least.

The point being that if you fill a large drum with steam and then close all the ports, it’s going to cool down and pull a strong vacuum as the water condenses.

No matter how hard the vacuum is though, the pressure differential is just 1 bar at maximum, which goes to show how much force just 1 bar of pressure can generate. If the container isn’t well symmetrical, the uneven forces act like levers that buckle the container walls.

Depending on the surface, I suggest 1/2″ Polycarbonate (Lexan is one brand of PC) at minimum. Figure the atmosphere is going to exert roughly 14.9 PSI on every square inch of material. PC is quite strong and it doesn’t fail as suddenly as acrylic does but if you are going to go over 6″ in diameter, check your numbers to ensure you have enough strength + a safety factor.

Absolutely. I didn’t actually run the numbers on this, but did everything I could to make sure it was “safe enough” – I’ve seen designs with the vacuum port on an acrylic lid, which made me shudder.

In terms of forces, a perfectly round porthole is the best you can get, the pan’s wall thickness is fairly well supported by both the bottom rim and the top rim that’s pressed very firmly against the lexan sheet.

Do note though that the difference between 0.00007 atm or 0.0000000000000000000001 atm is almost 0 psi. Maximum vacuum results in an exertion of about 14.9 PSI against the vacuum no matter how perfect of a vacuum you have – the atmosphere simply can’t “weigh” more than that.

Feel free to use that formula for future reference on “porthole” like designs under either vacuum or pressurized conditions. Notably thick portholes may require a modification due to assumptions about plane strain.

Both PMMA and polycarb have about the same tensile strength of ~50 MPa and elastic modulus. The only difference between the two is that polycarb has about 4 times the impact strength. Assuming the diameter of the pressure cooker is 30 cm he has about a 2x safety factor.

Partial vacuum has been good enougg for the few times I’ve cast small pieces. I was making silicone molds out of SmoothSil something or other.

Like a good HaD’er, I found an unused plastic canister in my kitchen, drilled a hole in the top and used my wine bottle vacuum [http://www.vacuvin.com/270/Vacuum_Wine_Saver.html]. Yeah, I suppose it could have imploded. But it didn’t.

This is like saying you need to mow your lawn with a 10 megawatt laser; sure it will do the job but is completely unnecessary.

To even run a turbo pump you needed a backing roughing pump, something like a rotary vane pump for instance. The roughing pump already can draw a good enough vacuum to serve your needs, you would just use that.

Also good look getting 10^-9 with a random metal pressure cooker, giant plastic porthole, and a big ‘ol lump of resin all of which will be outgassing like crazy.

Assuming that I didn’t need the viewing window, is it not possible to just use the top from the pressure cooker? Maybe not the particular pressure cooker that he used, but one of the more substantial ones with a machined aluminum top.